Study authors Dr. Samantha Joye (L) and Mary-Kate Rogener (R) in front of ALVIN, a manned deep-diving research submarine, during a 2014 Gulf of Mexico expedition. Photo courtesy of Dr. Andreas Teske, University of North Carolina Chapel Hill (ECOGIG).

Scientists conducted a time-series investigation of methane concentrations, oxidation rates, and rate constant (the instantaneous capacity of the methanotrophic community to consume methane) to describe methane dynamics after the oil spill. They combined this data with data collected before and during the spill for a more complete understanding of methane cycling in the system. Methane concentrations at all study locations were higher during the spill, indicating the dispersion of methanotrophic biomass away from the source point, and then decreased to near background conditions within a year. The relatively short-term methanotrophic bloom was followed by somewhat elevated methanotrophic activity or methane oxidation rates throughout 2012 and gradually returned to historical levels by 2015. The high oxidation rates caused a depletion of nitrate and phosphate near the wellhead. These results suggest that Gulf-wide circulation disbursed and redistributed the methanotrophic biomass bloom, which was associated with the persistent sub-surface oil plume, and perpetuated elevated methanotrophic activity for several years.

Methane is a potent greenhouse gas with global warming potential 28 times that of carbon dioxide, and in the ocean, it is consumed primarily through oxidation within sediments and in the water column. Deepwater Horizon released approximately 250,000 metric tons of low molecular weight alkane gases, mainly methane, resulting in the highest aerobic methane oxidation rates ever reported in an open ocean. The sudden large-scale release provided an opportunity to study microbial responses to large methane perturbations and to better understand potential effects on ecosystem-scale methane cycling.

The research team collected ~900 measurements of methane concentrations, methane oxidation rates, rate constants, hydrography, and nutrient availability from 31 sites during 2001, 2006, and 2010 – 2015. These sites included two natural seeps, one mud volcano, and one non-seep site near the Macondo wellhead. The resulting dataset represents the most extensive compilation of data on methane dynamics for a pelagic ocean ecosystem.

Natural seeps exhibited vertical methane profile structures with concentrations ranging between 10 – 250 nM, and the highest concentrations were at the seabed near the source. In contrast, samples collected during the spill exhibited a vertical and horizontal profile with highest concentrations in the deep-water plume well above the seabed. Methane concentrations associated with the oil plume were 10 – 103 times above background levels up to 381 μM during and after the spill and remained high for the rest of the 2010. Methane oxidation rates inflated from the background average of 60±146 to 5900 nmol L−1 d−1 during the spill and then slowly decreased over a five-year period.

There was no correlation between methane concentration and rate constant, suggesting that other factors such as variable circulation patterns, nutrient dynamics, and natural methane inputs influenced methanotrophic activity as much or more than methane concentration.

The authors noted that the long-term intensity of the oil spill led to the plume’s unprecedented spatial scale and temporal persistence, which provided the methane concentrations and time necessary for large-scale production of methanotrophic biomass. They also noted that the risk for future releases highlights the importance of long-term monitoring to assess how the ecosystem may respond.

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.